The iliacus is a flat, triangular muscle that covers the inner curve of the largest hipbone, or iliac fossa. It is a part of the inner hip muscle group, called the iliopsoas, or "IlioPsoas." In humans, the iliopsoas group of muscular tissue is important for standing, walking, running, and rotating the pelvis, or "hip flexion." Of the five muscles that contribute to hip flexion, the iliacus is the only one attached to the pelvis; it plays a major role, therefore, in controlling the hip joint and is important for flexing the leg forward.

The iliopsoas is a grouping of three muscles: the iliacus, the psoas major, and the psoas minor. Usually people refer to this group as the "dorsal hip muscles" or "inner hip muscles." The iliacus lies behind the psoas major. It starts at the upper two-thirds of the iliac fossa and at the inner rim of the curved ridge at the top of the pelvic bone, or iliac crest. It joins the psoas major, continues down, and attaches to the lesser trochanter of the upper femur.

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This attachment between the hipbone and leg bone is a major part of hip movement. Hip flexion uses five muscles — the tensor fascia latae, the rectus femoris, the sartorius, the major psoas and the iliacus. Of these five muscles, only the psoas and the iliacus are able to lift the upper leg to the torso, which is often necessary in sports, exercises, and dancing.

Frequently, sedentary lifestyles weaken the these muscles or cause them to tighten and become inflexible. Sometimes runners who do not start with warm-up exercises or run improperly can have weak or tight iliopsoas hip flexors as well. Typically, weak iliopsoas muscles offer poor back support and tight muscles may cause the pelvis to tilt forward, creating a swayback posture.

About five percent of soccer players injure this muscle, since it controls the hip movement when the player is twisting his hips during kicks. Other activities that are ripe for these injuries include bending, jumping, running, and squatting. Most injured players experience tendinitis, bursitis, or tears of the muscle. Typically, physical therapy includes exercises such as supine knee folds, Pilate's exercises, and various stretches. Usually, surgical treatment is rare.

Sometimes, medical professionals such as physical therapists and doctors describe the iliopsoas as "complex" and call it "the hidden prankster." For example, just as only about 40 percent of people have the psoas minor muscle, some people have an extra muscle — the iliacus minor or iliocapsularis. This muscle is a tiny, separate part of the larger muscle.

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